1. Field of the Invention
This invention relates to video recorders, especially those which employ magnetic tape as the recording medium, and which, on playback, feed video signals, say, to the input terminals of a television set. The term "recorder" as used herein shall be interpreted to include playback apparatus as well.
2. Description Relative to the Prior Art
As is known, the video spectrum, as typified by the NTSC baseband signal, contains frequency components from DC to 3 MHz or more, the lower frequencies corresponding at least in part to scene background information, and the higher frequencies corresponding to scene detail information. Because the playback of information recorded on a magnetic medium such as magnetic tape is frequency-dependent (i.e. system response to the playback signal is dependent on the rate of change of flux as seen by a playback head), flux patterns which correspond to low frequency signal components are not easy to detect in the magnetic tape. Prior attempts to record and play back, say, the baseband NTSC signal have, therefore, usually involved the baseband-modulation, during recording, of a relatively high frequency carrier. By so using a baseband-modulated carrier, the playback circuits of the recorder can be tuned to respond to the rapidly changing flux characteristics which correspond to the recorded carrier, with the baseband video signal being then relatively easy to detect and remove from the modulated carrier. While the practice of recording a modulated carrier solves the problem associated with the playback of low frequency information, it does so not without cost: First, because a high frequency carrier must be recovered during playback, the gap in the playback head--for a given speed of tape past the playback head--must be narrower than for the case of a directly recorded baseband signal, a factor which usually means higher head cost. And, second, because the baseband signal, when it modulates the carrier during the recording operation, produces signal sidebands--which must be recorded, and recovered during playback--the spectrum of frequencies to be processed by the recorder (i.e. the recorder bandpass requirements) will be wider than in the case of a baseband recorder. Such bandwidth requirements directly affect information packing density; and, thus, for maximum tape economy for a given amount of video information, the prior practice of recording a modulated carrier leaves much to be desired.
As indicated above, playback of magnetically recorded information depends on the rate of change of flux as seen by a magnetic head. This may suggest that--rather than place the baseband video signal on a carrier as a vehicle for playing back low frequency video--the track containing the recorded video information be widened a bit for cooperation with a widened magnetic playback head, the recorded information being the full video baseband signal. In that way, flux-to-head linkage will increase and, attendantly, so will the playback signal-to-noise ratio (SNR)--which should improve the recoverability of low frequency information. But widening the record track works counter to the goal of "high-packing-density tape usage". Besides, low frequencies, when recorded on a magnetic medium, create flux patterns within the medium which tend to spread out and, to assure that the low frequencies recorded in one track are not played back during the playback of an adjacent track, sizable guard bands between the tracks have to be used, again adversely influencing the efficient use of the magnetic medium.
Aside from the above techniques for recovery of low frequency information from a magnetic recording medium, one might also consider separating the low frequency components of the baseband signal from the higher frequency components, and then recording the low frequencies on a carrier in a separate track. But the use of two tracks to record the baseband information obviously is not in the interest of tape efficiency.
In copending application Ser. No. 669,925, filed Mar. 24, 1976 in the name of Edward M. Granger, now U.S. Pat. No. 4,104,683 baseband recording of the NTSC signal is indicated as being implemented by splitting the baseband signal into high and low frequency bands, the low frequency band being then time compressed and recorded analog-wise as a substitute for the sync and blanking information contained in the NTSC signal. Such a technique will work well so long as tape and other noise is relatively low, for low frequency SNR is extremely critical in the faithful reproduction of recorded scene information.